Nasa says its Mars rover Opportunity has shown unequivocally that the Red Planet had the right conditions to support life some time in its history.

The rover has revealed the rocks at its landing site were once in contact with substantial amounts of liquid water.

"These rocks were modified in liquid water and may have been precipitated in water," said scientist Steve Squyres.

Opportunity has been studying the local geology at a location called Meridiani Planum since its landing on 25 January.

Professor Squyres, the principal investigator on the rover's science payload, said his team had been engaged in a fine analysis of a section of layered bedrock at the landing site.

"For the last two weeks we've been attacking it with every piece of our hardware and the puzzle pieces have been falling into place," he told a special news conference at the US space agency's headquarters in Washington DC.

"Were these rocks acted upon, were they altered by liquid water? The answer to that question is, definitively, yes."

Rich in sulphur

He said there were several key lines of evidence to support the conclusion.

These included the rocks' physical appearance. Their cross-bedding, the presence of small spherules and indentations all pointed to water modification.

The rover's instruments also detected high levels of sulphate salts which on Earth would normally form in water or, after formation, be highly altered by long exposures to water.

Nasa has two rovers on opposite sides of Mars

"The only way you can form such large concentrations of salt is dissolve it in water and allow the water to evaporate," said mission scientist Dr Benton Clark.

In particular, the rover found jarosite, an iron sulphate mineral which suggests an acid-rich lake or hot-spring environment might have existed at Meridiani Planum.

"The purpose of this mission was to go to Mars and see if it had habitable environments," said Professor Squyres.

"We believe at this place on Mars for some period in time... this was a ground water environment that would have been suitable for life. That doesn't mean that life was there. We don't know that."

Rock return

The scientists still have to show the rocks were originally laid down by minerals precipitating out of solution at the bottom of a salty lake or sea - that they were formed like the water-derived sedimentary rocks found on Earth.

Nasa's scientists said it was not possible to say when the wet environment at Meridiani Planum existed.

Nasa believes the spherical granules have a watery origin

Neither the Opportunity rover, nor its twin, Spirit, is equipped to date rocks.

"The best way to get at the age is going to be to bring some of this stuff back," said Professor Squyres.

And Nasa officials believe a sample return mission should now be a priority.

"One of the pathways of exploration... is to undertake perhaps the most challenging robotic science mission we could imagine to another world - and that is to return pieces of Mars to Earth," said Dr Jim Garvin, the lead scientist for Mars and the Moon at the space agency's headquarters.

"These first results are a good compass point that says 'we know a good place go and get Mars, figure it out on site like we're doing and bring it home to all those [Earth] labs'."

He said a future mission could involve a rover that scoured the surface for interesting rocks which it then took back to a mothership for despatch to Earth.

Opportunity and Spirit are controlled by a team of scientists and engineers working out of the Jet Propulsion Laboratory in Pasadena, California.

Spirit touched down in Gusev Crater on the far side of Mars to Opportunity on 4 January. It is investigating an area which scientists think may once have held a lake.